Television and like reproducer



Dec. 2, 1941. 1.. M. MYERS TELEVISION AND LIKE REPRODUGER Filed Aug. 28, 1937 SECOND T/ME I NV EN TOR. L 50mm MORRIS MYERS A TTORN E Y.

mmc'r VOLT/16E m a l EQR RE w @atented Dec. 2, l94l TELEVISION AND LIKE anrnonnona Leonard Morris Myers, Middlesbrough, England, assignor to Radio C'q f oration of America, a

corporation of Delaware Application August 28, 1937, Serial No. 161,505 In Great Britain August 10, 1936 16 Claims. (01. 178-75) This invention relates to television and like reproducer and has for its object to provide an improved television or like reproducing apparatus capable of producing pictures of relatively large dimensions and light intensity suitable for projection on to a viewing screen.

According to this invention a television or like reproducer comprises a cathode ray beam apparatus wherein received television or like pictures are reproduced upon a screen by scanning it with a picture signal modulated cathode ray beam which heats individual elemental areas of said screen so as to cause light emission therefrom as a result of heating and the said invention is characterized in that heating of the aid elemental areas of said screen is continued during periods between instants of cathode ray passage by secondary electron bombardment which is initiated by said passage but is arranged to continue after said passage. The modulation of the cathode ray beam may be effected in any way known per se, e. g. intensity and/or velocity modulation may be used.

The general form of cathode ray television reproducer as at present in common use consists of a cathode ray tube having a fluorescent screen which is scanned by a television signal modulated cathode ray beam. The light output obtainable from such a fluorescent screen is severely limited by practical considerations and relatively large pictures of good fidelity are, practically speaking, unobtainable with such known cathode ray beam reproducers. If in place of a fluorescent screen a metallic screen is employed and the reproduced pictures are built up by causing localised light emission from said metallic screen as a result of heating the said screen by bombardment by the modulated scanning cathode ray beam a considerably greater light output can be obtained. On the other hand, whereas the response of a fluorescent screen to electron bombardment is almost immediate, there being very little lag, the difficulty a to lag is much more serious in the case of a metallic screen which is required to emit light as a result of heating by electron bombardment and in its broadest aspect the present invention meets this difliculty by causing the individual elemental areas of a screen to be bombarded by secondary electron during the intervals between successive passages of the cathode ray beam, this secondary electron bombardment being initiated by the passage of the cathode ray beam and being of an intensity dependent, inter alia, upon the required picture point brightness of the said elemental areas.

The invention is illustrated in and further explained in connection with the accompanying diagrammatic and graphical drawing in which Fig. 1 shows diagrammatically one embodiment of my invention embracing an electron gun defleeting system and secondary electron multiplier; Figs. 2 and 3 show graphically relationships between impact voltage and electron multiplication and light output as a function of time respectively; while Figs. 4 and 5 show further modifications of the embodiment shown in Fig. 1.

Referring to Figure 1 which illustrates the principle of the invention by showing, purely diagrammatically, one way of carrying the invention into effect there is employed a cathode ray tube having an electron gun represented at l and associated mutually perpendicular pairs of deflecting plates 2, 3, for projecting and electrostatically deflecting a cathode ray beam represented by the chain line 4 the beam being thus projected upon and caused to scan a metallic screen 5 composed of tungsten particles embedded in carbon. Obviously elcctro-magnetic deflection may be used in place of electrostatic deflection or one component of deflection movement may be obtained electromagnetically and the other electrostatically. The screen may be constituted by a mixture of tungsten powder with colloidal graphite the said mixture being formed as a paste and pressed into the interstices of a fine tungsten mesh of, for example, 400 to the inch. The liquid used for forming the paste preferably includes potassium silicate or sodium silicate as the use of these materials results in an efficient binder when the water is dried out. In front of the screen 5, that is to say on the side thereof facing the electron gun and deflecting system, is a mesh electrode 6 of tungsten wire, the mesh electrode t and metallic screen 5 being parallel to one another and spaced a short distance apart, for example 2 or 3 cms. It will be appreciated that the tungsten-carbon screen structure described is of low thermal conductivity, good electrical conductivity, and is capable of withstanding a high temperature, in fact a temperature of about 3,000" K.

In use the cathode of the electron gun is maintained at a potential well below that of the metaliic screenfor example, it may be connected to a point of -3000 volts potentialand a high frequency oscillatory voltage wave is applied between the metallic screen and the mesh electrode. The amplitude of the high frequency Wave may be about 300 volts and the frequency may correspond to a wave length of the order of 3 metres or thereabouts. In Figure l the high frequency wave is applied from a suitable oscillator ll via a coil 1 having an earthed centre point and connected between electrodes 5 and 6. A magnetic focussing coil 8 extending at least from the metallic screen 5 to the electrode 6 surrounds the envelope l3 of the tube and is fed with direct current so that it applies an intense magnetic field at right angles to the parallel planes in which the electrodes 5 and 5 lie.

In operation the metallic screen 5 is scanned by the electron beam projected from the gun I and modulated as to intensity as in the usual way, the electrons passing through the interstices in the mesh electrode 6 to the metallic screen 5. Owing to the high velocity of the electrons in the cathode ray beam-this velocitycan be varied at will for optimum operating conditionssecondary emission will take place from any point in the metallic screen when it is struck by the scanning cathode ray beam and the intensity of the secondary emission from the said point will clearly depend upon the intensity of the cathode ray beam striking it. It will thus be clear that during one scan of the metallic screen there will be formed what may be termed a secondary emission image corresponding to the picture to be reproduced. Consider the action at some particular point in this said image. As stated, secondary electrons will be given ofi from said point with an intensity depending upon the intensity of the cathode ray beam bombarding that point. Owing to the alternating potential wave applied between the metallic screen 5 and the mesh electrode 6 these secondary electrons will proceed to the mesh electrode 6 where they will liberate secondary electrons which will proceed back to the metallic screen 5 to liberate further secondary electrons, and so on. Thus each point in the metallic screen 5 will continue to be bombarded by secondary electrons after the scanning cathode ray beam has passed the said point and by correct choice of the electrical parameters (and notably of the.

high frequency oscillatory potential and of the bombardment of that point continues at a diminu? ishing intensity falling substantially or nearly to zeroby the time the cathode ray beam again passes the said point. It will be appreciated that it is possible by causing too intense a secondary emission for the metallic screen and associated mesh electrodes to be burned, and this condition should, of course be avoided. The coil 8 surrounding the space between the metallic screen 5 and the mesh electrode 6' provides focussing for the secondary electrons.

In any particular case the selection of the best operating point is probably most easily found by trial and error, but the graphical Figures 2 and 3 will be of assistance in understanding the conditions to be satisfied for best results. Figure 2 is a typical curve connecting electron multiplication (number of emitted secondary electrons from the screen 5 as above described per incident electron) (ordinates) with electron impact voltage (abscissae). As will be seen there are two points X, Y, corresponding to about 900 volts and 3,000 volts, where the multiplication is unity, and one of these points should be selected as the operating point. If the multiplication exceeds unity there is danger of burning of the screen,

assuming that the oscillator is of sufiicient power. Since the multiplication is chosen at substantially unity or less than unity, the stream of electrons impinging on any element in the screen becomes less in course of time after the scenning ray or beam has passed that element and the operating data are so chosen that the current falls to a small value, at which no visible light emission occurs in one picture period or slightly less. This is illustrated by the full line curve in Figure 3 in which visible light output (ordinates) is. plotted against time (abscissa) for a case where. the picture period is second. For purposes of comparison there is shown by a broken line in Figure 3 the corresponding lighttime curve for an ordinary fluorescent screen type of cathode ray receiver tube.

It will be noted that in the above described arrangement the power for the production of the light in the reproduced pictures is derived from the oscillatory source instead of from the scanning cathode ray beam.

If the metallic screen 5 and the mesh electrode 5 are too far apart the alternating voltage required to overcome space charge effects becomes excessive so that these two electrodes should be relatively close together. Alternatively, however, a grid (not shown) may be introduced between these two electrodes thi grid receiving a high positive potential with respect to the two said electrodes (ignoring the alternating potential between on these electrodes) and serving to reduce the space charge.

A defect of the arrangement of Figure 1 is that due to the presence of the intense magnetic field there is considerable difficulty in obtaining a properly focussed cathode ray spot uponthe metallic screen. This defect may be avoided as shown in Figure 4 by replacing the mesh electrode 5' of Figure 1 by a pair of co-axial cylindrical electrodes 5a 617 having their common axis perpendicular to the plane of the metallic screen and being of about the same diameter as the metallic screen 5. The high frequency oscillatory voltage wave is now applied between the metallic screen 5 and the cylindrical electrode 6a. which is nearest thereto, the centre point of the coil I through which the high frequency voltage wave is applied, being earthed and a suitable negative bias-say 1000 volts-being applied to the remaining cylindrical electrode 6b. The coil 8 extends at least from electrode 5 to electrode 62).

The action of the arrangement of Figure 4 is as follows:

First the scanning beam liberates secondary electrons from some point in the metallic screen 5. This emission may be assumed to occur when the metallic screen 5 and the cylindrical electrode 6a are at zero potential. Immediately thereafter the potential of the electrode 6a starts to rise andthat of the metallic screen 5 to fall. Therefore electrons travel towards the electrode to but, owing to its increasing positive potential overshoot it until they are retarded by the electrostatic field from the negatively biased electrode 5b. This negatively biased electrode 611 returns the electrons back towards the metallic screen 5 and by this time the relative potential between the metallic screen 5 and the adjacent cylindrical electrode So has become reversed in sense so that the returning electrons are now accelerated through the electrode to and strike the screen 5 with sufficient velocity to cause the emission of further secondary electrons which are subjected to a like process.

The intense magnetic field is applied in order to cause the secondary electrons to travel in helice of radius not greater than the radius of a picture element. In practice an applied field in the neighbourhood of 6,000 gauss is suitable. As before the parameters of the arrangement are so selected that during one frame period secondary electron emission from any point reduces almost to zero and again, as before, the said parameters are chosen to avoid risk of destructicn of the electrodes by over-intense electron bombardment.

Figure shows a modification of the arrangement of Figure 4 the modification consisting of what may be termed a resistance lens 9 (represented in broken lines) between the oscillating electrodes 5, 6a, 6b. This lens i of high resistance and may be constituted by a thin coating of colloidal graphite on the inside of an insulating tube (not shown) of silica or the glass known under the registered trade-mark, Pyrex, or other suitable material, said tube also containing and supporting the electrode 5, 6a, 6b. The lens 9 serves to reduce space charge effects and thus to increase the current between the electrodes.

Pictures reproduced by apparatus as above described may be projected by any suitable optical system, upon a viewing screen.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed we declare that what we claim is:

1. The method of reproducing images which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward an electron sensitive surface to impact the beam thereon to both produce light energy and to emit secondary electrons, multiplying the emitted secondary electrons, and directing the multiplied electrons to impact upon the said surface.

2. The method of reproducing images, which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward a light producing and secondary electron emitting surface, scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, multiplying the emitted secondary electrons, and directing the multiplied electrons to impact upon the said surface.

3. The method of reproducing images, which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward a light producing and secondary electron emitting surface, scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, multiplying the emitted secondary electrons, and directing the multiplied electrons to impact upon the said surface to emit more light energy and more secondary electrons, each of a diminished intensity compared with the intensity of the light energy and secondary electrons emitted under the impact of the produced beam.

4. The method of reproducing images, which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward a light producing and secondary electron emitting surface, scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, multiplying the emitted secondary electrons, and directing the multiplied electrons to impact upon the said surface to emit more light energy and more secondary electrons, each of a diminished intensity compared with the intensity of the light energy and secondary electrons emitted under the impact of the produced beam, multiplying the secondary electrons emitted by the directed secondary electrons, and directing the last of said multiplied secondary electron to impact on the said surface with further diminishing of the intensity of both the light and secondary electrons produced.

5. The method of reproducing images, which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward a light producing and secondary electron emitting surface, scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, multiplying the emitted secondary electrons, directing the multiplied electrons to impact upon the said surface to emit more light energy and more secondary electrons, each of a diminished intensity compared with the intensity of the light energy and secondary electrons emitted under the impact of the produced beam, multiplying the secondary electrons emitted by the directed secondary electrons, directing the last of said multiplied secondary electrons to impact on the said surface with further diminishing of the intensity of both the light and secondary electrons produced, and repeating the steps of multiplying the emitted secondary electrons, and directing the multiplied electrons toward the surface until the intensity of secondary electrons is reduced substantially to zero.

6. The method of reproducing images, which includes the steps of producing a concentrated beam of electrons, directing the produced beam toward a light producing and secondary electron emitting surface, scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electron are emitted from the said surface, multiplying the emitted secondary electrons, directing the multiplied electrons to impact upon the said surface to emit more light energy and more secondary electrons, each of a diminished intensity compared with the intensity of the light energy and secondary electrons emitted under the impact of the produced beam, repeating the successive steps of multiplying the emitted secondary electrons, and directing said electrons toward the surface until the emitted secondary electrons become substantially zero within a predetermined time duration.

7. A cathode ray tube comprising means for producing a concentrated beam of electrons, means for directing the produced beam toward an electron sensitive surface to impact the beam thereon to both produce light energy and to emit secondary electrons, means for multiplying the emitted secondary electrons, and means for directing the multiplied electrons to impact upon the said surface.

8. A cathode ray tube comprising means for producing a concentrated beam of electrons, means for directing the produced beam toward a light producing and secondary electron emitting surface, mean for scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, means for multiplying the emitted secondary electrons, and means for directing the multiplied electrons to impact upon the said surface.

9. A cathode ray tube comprising means for producing a concentrated .beam of electrons, means for directing the produced beam toward a light producing and secondary electron emitting surface, means for scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary elec trons are emitted from the said surface, means for multiplying the emitted secondary electrons, and means for directing the multiplied electrons to impact upon the said surface to emit more light energy and more secondary electrons, each of a diminished intensity compared with the intensity of the light energy and secondary electrons emitted under the impact of the produced beam.

10. A cathode ray tube comprising means for producing a concentrated beam of electrons, means for directing the produced beam toward a light producing and secondary electron emitting surface, means for scanning the surface by the 1 produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, means for repeatedly successively multiplying emitted secondary electrons and for directing the multiplied electrons to impact on said surface to emit additional light, and further secondary electrons for a predetermined time interval.

11. A cathode ray tube comprising means for producing a concentrated beam of electrons, means for directing the produced beam toward a light producing and secondary electron emitting surface, means for scanning the surface by the produced beam according to a predetermined pattern whereby light energy and secondary electrons are emitted from the said surface, means for repeatedly successively multiplying emitted secondary electrons for directing the multiplied electrons to impact on said surface to emit additional light and further secondary electrons until the number of electrons is reduced to zero, the multiplication factor of said successive multiplication of electrons having a maximum value of unity.

12. The method of reproducing images which includes the steps of producing a concentrated beam of electrons, controlling the intensity of the produced beam of electrons in accordance with electrical signals representative of an image to be reproduced, directing the produced beam toward an electron sensitive surface to impact the beam thereon to both produce light energy and to emit secondary electrons, multiplying the emitted secondary electrons, and directing the multiplied electrons to impact upon the said surface.

13. A cathode ray tube comprising means for producing a concentrated beam of electrons, means for controlling the intensity of the produced beam of electrons in accordance with electrical signals representative of an image to be reproduced, means for directing the produced beam toward an electron sensitive surface to impact the beam thereon to both produce light energy and .to emit secondary electrons, means for multiplying the emitted secondary electrons, and means for directing the multiplied electrons to impact upon the said surface.

14. A cathode ray tube comprising an electron gun for producing a beam of electrons, means to deflect the beam of electrons in mutually perpendicular directions, an electron multiplier having an electrode whose surface is adapted to emit :both light and secondary electrons under the impact of the produced beam of electrons, an elec-- trode intermediate the said surface and beam deflecting means, focusing means intermediate the said surface and electrode, and means to supply alternating potential between the said surface and electrode.

15. A cathode ray tube comprising an electron gun for producing a beam of electrons, means to deflect the beam of electrons in mutually perpendicular directions, an electron multiplier having an electrode whose surface is adapted to emit both light and secondary electrons under the impact of the produced beam of electrons, an electrode intermediate the said surface and beam defleeting means, focusing means intermediate the said surface and electrode, means to supply alternating potential between the said surface and electrode, an electrode intermediate the first electrode and deflecting means, and means to maintain the second named electrode negative with respect to the first electrode.

16. A cathode ray tube comprising an electron gun for producing a beam of electrons, means to deflect the beam of electrons-in mutually perpendicular directions, an electron multiplier having an electrode whose surface is adapted to emit both light and secondary electrons under the impact of the produced beam of electrons, an electrode intermediate the said surface and beam deflecting means, focusing means intermediate the said surface and electrode, means to supply alternating potential between the said surface and electrode, an electrode intermediate the first electrode and deflecting means, means to maintain the second named electrode negative with respect to the first electrode, and resistance means embracing thesaid surface and electrodes for maintaining a potential drop between the surface and the first named electrode and between the first named electrode and the second named electrode.

LEONARD MORRIS MYERS. 

